The present invention relates to a package for an electronic device and, in particular, to a package for an electronic device that includes a flexible substrate including a flexible dielectric adhesive having a low modulus of elasticity.
As semiconductor integrated circuit technology has advanced to greatly increase the amount and operating speed of the circuitry that can be fabricated on a single semiconductor chip, it has become more difficult to effectively utilize such integrated circuits due to the greatly increased number of input and output connections to the chip and the decreasing spacing or pitch of those connections. This was formerly less of a problem because semiconductor chips were mounted into mechanical packages, such as flat-packs and dual-in-line packages, that provided relatively widely-spaced and easily solderable or weldable leads, but were much larger than the semiconductor chip it contained. The connection problem has become more severe where the numbers of connections exceeds that conveniently or economically attainable in a conventional mechanical package.
One approach to solve this problem utilizes semiconductor chips mounted with contacts against and connecting to corresponding contacts on the next-level circuit board, the so-called xe2x80x9cflip-chipxe2x80x9d mounting. This flip-chip technique requires that the contacts on the next-level circuit board be of substantially the same size and of the same pitch as are those on the semiconductor chip. In particular, the pitch of the semiconductor chip connections has become much finer than the pitch attainable on conventional mechanical packages and printed wiring circuit boards to which such semiconductor chips are mounted. In addition, the differences in thermal expansion between the semiconductor chip and the next-level circuit board produces thermally-induced stress that leads to failure or degradation of the interconnections when exposed to thermal cycling, which stress is often exacerbated by the rigidity of the solder interconnections therebetween.
A further solution to these problems has employed an intermediate substrate between the semiconductor chip and the next-level circuit board to absorb some of the thermally-induced stress, and also to allow the fanning out of the connections to the semiconductor chip to permit a larger contact size and pitch that is compatible with conventional printed wiring circuit board technology. If the intermediate substrate is substantially larger than the size of the semiconductor chip, then the advantage of small chip size is lost, as is the advantage of short electrical lead length that improves the ability to operate the circuit at very high operating frequencies. While this has been addressed by reducing the size of the intermediate substrate and employing next-level substrate technologies capable of finer line widths and smaller features, the rigidity of the intermediate substrate has again posed some difficulties. Electronic packages where the perimeter of the intermediate substrate is no more than about 20% larger than the perimeter of the semiconductor chip mounted thereon are often referred to as xe2x80x9cchip scale packages,xe2x80x9d although larger packages are often also referred to as xe2x80x9cchip scale packages.xe2x80x9d
The difficulties of rigid intermediate substrates has been addressed by making the substrates of specialized materials that are referred to as being xe2x80x9cflexible,xe2x80x9d such as thin polyimide and other so-called xe2x80x9cflexiblexe2x80x9d conventional substrates on which printed wiring conductors and plated through holes can be formed by conventional methods. But, such substrate materials are not truly flexible in that they do not have a low modulus of elasticity, but only flex to a greater extent because they have been made of thinner material having a high modulus of elasticity. Conventional materials, such as polyimide sheet, have a high modulus of elasticity, e.g., a modulus greater than 70,000 kg/cm2 (1,000,000 psi). In addition, the use of such materials and conventional fabrication methods results in an increased cost that is undesirable and may require assembly processes that are more difficult or expensive to perform.
In addition, enclosed cavity packages are often preferred due to their resistance to the entry of moisture, such as the hermetically-sealed packages usually employed in high reliability, military, aerospace, and medical electronic applications, and in applications of optical devices and frequency-sensitive communication devices. Such packages are generally metal or ceramic with seals formed of glass or metal solders or brazing. The ability of a package to resist the entry of moisture, or to allow the easy exit of moisture, is of importance to reliability of operation. Typically, hermetic type packages are most reliable; lidded packages are less reliable than hermetic packages, but are more reliable than are glob-top, molded or encapsulated packages. In addition, a cavity-type package can offer advantage in that less mechanical stress is imposed on the semiconductor chip than is the case with glob-top or molded packaging, as is important for stress-sensitive components such as optical devices, sensors, frequency crystals, multiple-chip modules, and the like.
Conventional hermetic cavity type packages are very expensive, due to the metal and/or ceramic package, the slow methods utilized for sealing the rim of the package lid and high labor content. Lidded cavity packages are much less expensive than hermetic packages, but are still expensive as compared to encapsulated packages, such as the molded epoxy or molded plastic encapsulated packages, that are employed in about 95-99% of commercial electronic applications. Even glob-top encapsulated packaging is more expensive than molded packages due to the inherently slow process of dispensing precise amounts of encapsulant, even using precision dispensing equipment. Specifically, a hermetic cavity type package can have a material cost of US$0.90-1.00 or more and a conventional lidded package with dispensed adhesive can have a material cost of US$0.20-0.40, as compared to a molded epoxy package which can have a material cost of only US$0.01-0.05 (exclusive of tooling).
Accordingly, there is a need for an electronic package that is suitable for a high-density (e.g., chip-scale) package, and that avoids some of the technical disadvantage of conventional molded packages without the high cost of conventional hermetic packages.
To this end, the present invention comprises an electronic package having contacts adapted to be attached to a substrate, wherein the electronic package comprises at least one electronic device, said electronic device having a plurality of contacts thereon, a flexible dielectric adhesive interposer, and means for connecting the contacts of the electronic device to certain ones of the conductive vias. The flexible dielectric adhesive interposer includes at least one layer of flexible dielectric adhesive having a modulus of elasticity less than about 35,000 kg/cm2 (about 500,000 psi), a plurality of conductive vias through the layer of flexible dielectric adhesive, wherein at least certain ones of the plurality of conductive vias correspond to ones of the contacts of the electronic device, and a metal foil on one surface of the layer of flexible dielectric adhesive, wherein the metal foil is patterned and is in electrical contact with ones of the conductive vias. One of the plurality of conductive vias and the patterned metal foil includes contacts adapted to be attached to a substrate.
According to another aspect of the invention, an electronic package for plural electronic devices comprises a plurality of electronic devices, each electronic device having a pattern of a plurality of contacts thereon, a plurality of flexible dielectric adhesive interposers each associated with at least one of the plurality of electronic devices, and means for connecting the contacts of each of the plurality of electronic devices to certain ones of conductive vias of the flexible dielectric adhesive interposer with which it is associated. Each flexible dielectric adhesive interposer includes at least one layer of flexible dielectric adhesive having a modulus of elasticity less than about 35,000 kg/cm2 (about 500,000 psi), a plurality of conductive vias through the layer of flexible dielectric adhesive, wherein at least certain ones of the plurality of conductive vias correspond to ones of the contacts of the associated one of the plurality of electronic devices, and a metal foil on one surface of the layer of flexible dielectric adhesive, wherein the metal foil is patterned and is in electrical contact with ones of the conductive vias. At least one of the plurality of conductive vias and the patterned metal foil includes external contacts adapted for connecting the flexible dielectric adhesive interposer to an external device. The plurality of flexible dielectric adhesive interposers are positioned adjacent each other and further comprise means for connecting the external contacts of each of the plurality of flexible dielectric adhesive interposers to the external contacts of an adjacent one of the plurality of flexible dielectric adhesive interposers.
Further, a method of making an electronic package for an electronic device comprises:
providing a sheet of metal foil;
providing at least one layer of a flexible dielectric adhesive having a modulus of elasticity less than about 35,000 kg/cm2 (about 500,000 psi) on one surface of the sheet of metal foil, the layer of flexible dielectric adhesive having a plurality of via openings therein;
building up conductive material on the metal foil to fill the via openings, thereby forming conductive vias therein;
patterning the metal foil to form a pattern of contacts and conductors electrically connected to the conductive vias in the flexible dielectric adhesive layer;
plating at least one of the conductive vias and the contacts of the patterned metal foil to provide external contacts; and
electrically connecting contacts of at least one electronic device to corresponding ones of the conductive vias.